[libvirt] [PATCH] qemu: conf: Work around race condition on libvirt start

[Adam Walters]

This patch works around a race condition present during
libvirt startup. The race condition only applies when
using storage pool volumes for domain disks, and even
then, only when restarting libvirt with running domains.

The gist of the patch is simply to enter a (limited)
retry loop during qemuTranslateDiskSourcePool. This
particular implementation does have a slight drawback,
though. Within that function, I can not determine if
we are currently starting libvirt, or if we are just
starting up a domain. In the latter case, this could
cause a 800ms delay in reporting an error that the
storage pool is inactive.

I am happy to report, however, that with this patch,
domains continue to run without restarts regardless
of how often I restart libvirt. I have a fairly fast
hypervisor, and have never seen it try more than one
iteration of the retry loop unless I purposely set
one of the storage pools to be inactive.
---
 src/qemu/qemu_conf.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/qemu/qemu_conf.c b/src/qemu/qemu_conf.c
index c28908a..2e52fbf 100644
--- a/src/qemu/qemu_conf.c
+++ b/src/qemu/qemu_conf.c
@@ -1359,6 +1359,8 @@ qemuTranslateDiskSourcePool(virConnectPtr conn,
     virStorageVolInfo info;
     int ret = -1;
     virErrorPtr savedError = NULL;
+    int attempt = 0;
+    int poolAutostart;
 
     if (def->type != VIR_DOMAIN_DISK_TYPE_VOLUME)
         return 0;
@@ -1369,7 +1371,16 @@ qemuTranslateDiskSourcePool(virConnectPtr conn,
     if (!(pool = virStoragePoolLookupByName(conn, def->srcpool->pool)))
         return -1;
 
+retry:
     if (virStoragePoolIsActive(pool) != 1) {
+        if (!(virStoragePoolGetAutostart(pool, &poolAutostart) < 0))
+            if (poolAutostart && attempt < 4) {
+                VIR_DEBUG("Waiting for storage pool '%s' to activate",
+                          def->srcpool->pool);
+                usleep(200*1000); /* sleep 200ms */
+                attempt++;
+                goto retry;
+            }
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                        _("storage pool '%s' containing volume '%s' "
                          "is not active"),
-- 
1.8.4.2